Depending upon the cooling load a refrigeration system is required to chill, the cooling demand placed on the system can vary widely. For example, when the refrigeration system serves the chilling requirements of a beverage dispenser, customer demand for beverages can vary from no drinks dispensed per minute to as many as 3 or 4 or more drinks dispensed per minute. This volatile variation in customer demand results in a broad range of cooling load requirements for the refrigeration system. When no beverages are being dispensed, the maintenance cooling load of a beverage dispenser can be as low as about 1500 Btu/hr. At the other extreme and during periods of high drink draw rates, cooling requirements can exceed 18,000 Btu/hr.
On-off operation of a compressor of a refrigeration system for a beverage dispenser is conventionally controlled by fixed temperature set-points that define a permissible range of beverage temperatures, ideally so that the beverage does not get either too warm or too cold and so that the refrigeration system compressor is not cycled on/off excessively. One set-point represents a maximum upper temperature that the beverage is permitted to reach before being chilled and the other set-point defines a minimum lower temperature of the beverage to be dispensed. When the refrigeration system compressor is off, the upper set-point is that temperature to which the beverage is allowed to warm before the compressor is cut-in or turned on to chill the beverage and reduce its temperature. The lower set-point is then that temperature to which the beverage is chilled before the compressor is cut-out or turned off. One problem with this technique is that it is not energy efficient. To conserve energy, it would be advantageous if beverage temperatures were allowed to be warmer during non-business hours than during business hours, but that cannot be accomplished with fixed upper and lower set-point temperatures. Another problem with the conventional approach is that fixed beverage temperature set-points constrain the refrigeration system compressor to operation between fixed cut-in and cut-out temperatures, irrespective of whether there are minimum or maximum cooling load demands being placed on the system. Consequently, when there is a minimum cooling load demand the refrigeration system cannot operate in a mode to conserve energy, and when there is a maximum cooling load demand the system cannot operate at an increased capacity to ensure that beverages are always properly chilled.